1. Field of the Invention
The present invention relates to an image coding technique and, more particularly, to an image coding technique using motion compensation.
2. Description of the Related Art
In image coding schemes specified by standards such as H.26x, MPEG, and the like, one moving image frame is divided into a plurality of blocks, and coding is done for each block.
In these image coding schemes, a coding scheme (inter coding) that exploits temporal correlation of moving image frames, and a coding scheme (intra coding) that exploits spatial correlation in one frame can be used. The coding scheme that exploits temporal correlation searches for the position in a reference frame to which a coding target block moves, and encodes the difference between a motion vector that represents the amount of motion and direction, and the block at the motion destination, thus compressing the data amount. Such coding scheme that considers the motion of a coding target block is called a motion-compensated coding scheme.
In order to improve the coding efficiency in the motion-compensated coding scheme, the reference frame must be searched for an area with a largest correlation with the coding target block. However, upon enhancing the search precision, a calculation volume required for search becomes huge.
For example, a case will be examined below wherein coding is made in an image of 64×64 pixels/frame having gradation of 8 bits/pixel for respective blocks each including 16×16 pixels. In this case, upon conducting a full search that calculates a correlation value with a coding target block while shifting pixels one by one for the entire reference frame (64×64 pixels), 1048576 (=16×16×64×64) 8-bit comparison operations are required. In addition, since this calculation is made for each of a plurality of blocks of a coding target frame, a huge calculation volume is required.
In order to reduce the calculation volume, various schemes such as a motion search scheme using a binary image or N (M>N≧2)-arized image as a coding target image as an M (M>2)-arized image (for example, see JP-B-3028685, Jeng-Hung Luo, Chung-Neng Wang, and Tihao Chiang, “TA Novel All-Binary Motion Estimation (ABME) With Optimized Hardware Architectures”, IEEE Transactions On Circuits And Systems For Video Technology, Vol. 12, No. 8, August 2002) and the like have been proposed. Since a binarized image search uses a binary image as an input image, a comparison operation per pixel uses a 1-bit comparison operation in place of the 8-bit comparison operation, thus reducing the calculation volume.
However, with the conventional binarized image search, although the calculation volume is reduced, since the input image is binarized, the correlation calculation precision is low, and the search precision of a motion destination of a coding target block, that is, that of a motion vector drops. For this reason, this consequently results in a coding efficiency drop. In case of an N-arized image search scheme, higher precision is assured than the binarized image search scheme, but that scheme also lowers gradation of the input image, thus posing the same problem.